This application relates to the control of an engine of a motor vehicle and, in particular, to a method for controlling the operation of such an engine so as to reduce fuel usage. Motor vehicles and, in particular, commercial vehicles operate with a wide range of payloads. The engine of a vehicle is designed to provide adequate performance when the vehicle is fully laden. As a result, the engine will often have an excess of performance when the vehicle is in an unladen condition. Specifically, a powertrain capable of high torque may equate to high acceleration potential when the vehicle is unladen or lightly laden. However, this may result in driving the vehicle inefficiently at a loss to fuel economy.
The inventors have realized that by limiting the accelerative performance of the motor vehicle when the motor vehicle is in a part laden or unladen condition, a significant reduction in fuel usage can be obtained. It is an object of the application to provide a method and system for controlling the engine of a motor vehicle to reduce fuel usage without unduly compromising vehicle performance.
According to a first embodiment, there is provided a method for controlling an engine of a motor vehicle having an engine driven transmission providing a number of gear ratios, wherein the method comprises controlling the engine in response to a torque request, setting an acceleration limit based upon at least a current gear ratio, estimating a current road load for the motor vehicle, and setting a maximum torque request limit based upon the estimated road load and the acceleration limit.
The acceleration limit may be based upon the current gear ratio and the current rotational speed of the engine. Estimating the current road load may comprise determining a current torque output from the engine, determining the acceleration of the motor vehicle due to the current torque output, and using the values of acceleration and torque output to estimate road load.
The method may further comprise reducing the torque request from a demanded torque request to the maximum torque request limit if the demanded torque request exceeds the maximum torque request limit.
The method may further comprise reducing the torque request from the demanded torque request to a modified torque request if the demanded torque request exceeds a torque request limit that is lower than the maximum torque request limit.
The torque request limit may be related to the maximum torque request limit. The torque request limit may be one of a predefined percentage and a predefined fraction of the maximum torque request limit. If the demanded torque request is less than the torque request limit, the torque request to the engine may be the same as the demanded torque request. The demanded torque request may be a torque demand from a driver of the motor vehicle. Alternatively, the demanded torque request may be a torque demand from a cruise control of the motor vehicle.
According to a second embodiment, there is provided a system for controlling the operation of an engine of a motor vehicle having an engine driven transmission providing a number of gear ratios, the system comprising a main electronic processor arranged to receive information regarding the current operating state of the engine and the current operating state of the motor vehicle including current gear ratio and an engine control processor to control the operation of the engine in response to a torque request from the main processor wherein the main processor is operable to set an acceleration limit for the motor vehicle based upon at least the current gear ratio, estimate a current road load for the motor vehicle and set a maximum torque request limit based upon the estimated road load and the acceleration limit.
The main processor may be arranged to receive information regarding the current rotational speed of the engine and the current gear ratio and may be operable to set the acceleration limit for the motor vehicle based upon the current rotational speed of the engine and the current gear ratio.
If a demanded torque request is greater than the maximum torque request limit, the main processor may be operable to reduce the demanded torque request to the maximum torque request limit. The main processor may be further operable to reduce the demanded torque request to a modified torque request if the demanded torque request exceeds a torque request limit that is lower than the maximum torque request limit. The torque request limit may be based upon the maximum torque request limit. The torque request limit may be one of a predefined percentage and a predefined fraction of the maximum torque request limit. If the demanded torque request is less than the torque request limit, the main processor may be operable to supply a torque request to the engine control processor that is the same as the demanded torque request. The demanded torque request may be one of a torque demand from a driver of the motor vehicle and a torque request from a cruise control of the motor vehicle.